|MadSci Network: Genetics|
Dear Mark, This is quite a difficult one to answer. Mendel's principles (Mendel of the sweet pea fame) were concerned the inheritance of genes but there is more to the story than that. I think what you need to do is have a look at the subjects of Penetrance and expressivity. Penetrance is the percentage of individuals with a particular genotype who have the phenotype associated with that genotype. If individuals have a particular gene they may still not express it if they have supressors, modifiers or epistatic genes in the rest of the genome. Penetrance can be used to measure such effects when the cause of the modification are unidentified. Expressivity measures the extent to which a given genotype is expressed in the individual at the phenotypic level. Factors which effect this may be due to the environemt or because of differences in the allelic contitution of the rest of the genome. These two things can often make it impossible to make predictions in human pedigree analysis. You've really got to consider Mendel's inheritance and patterns of expression. You can have Complete dominance (Mendel's situation)Dominant homozygous and heterozygous look the same. This is one extreme of a spectrum. The other end of this spectrum is co-dominance . You must remember that this is not evidence of blending though. Incomplete dominance is where you get intermediate phenotype heterozygotes. Some traits depend on multiple alleles e.g. height. Now, the term 'race' doesn't really mean much in genetics. This is because in our ancestry it is impossible to know when peoples met etc. It is better just to deal with individuals and their genes. I tried to look this up for you but I coldn't find much about eye colour. As far as I know brown eye alleles are dominant but I suspect there is more to this since that is never used as an illustration of Mendelian inheritance in the books I read. Skin colour is polygenic so it would be difficult to predict from skin colour what colour a person's eyes would be - you're dealing with difference alleles which are independent. Think how skin colour can chance in the sun and you'll see some of the complications. I hope I've given you a starting point for further research. This story simply isn't a simple one. Co- dominance
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